Emission control devices, such as a three-way catalyst, coupled to an exhaust of a combustion engine reduce combustion by-products such as carbon monoxide, hydrocarbons, and oxides of nitrogen. To reduce emissions, catalyst monitoring methods may be used to detect when an emission control device has reached its threshold use and is to be replaced. Reliable catalyst monitoring may reduce costs by decreasing erroneous characterization of a useful catalyst as an expended catalyst, or may reduce emissions by decreasing erroneous characterization of a degraded catalyst as a useful catalyst.
Various approaches for catalyst monitoring have been developed including methods provided for monitoring an emission control device comprising following a deceleration fuel shut-off duration, indicating degradation of the emission control device based on an integrated air-fuel ratio method and a steady-state based index ratio diagnostic method. As such, the integrated air-fuel ratio method estimates an amount of fuel mass that may be consumed to react with stored oxygen in the catalyst for a post-catalyst sensor breakthrough.
The inventors herein have recognized an issue with the above approach. Namely, the integrated air-fuel ratio method may be more sensitive to noise in measurement of air-fuel ratio. For example, the integrated air-fuel ratio method uses a calibrated air-fuel ratio in the calculation until a pre-catalyst gas sensor reaches stoichiometry, instead of a measured air-fuel ratio from the pre-catalyst gas sensor. Additional noise may be introduced in the calculation by using a constant value for the calibrated air-fuel ratio.
One approach that at least partially addresses the above issue includes a method comprising, following a deceleration fuel shut-off (DFSO) duration, indicating degradation of an emission device based on an amount of rich products required to cause a sensor to become richer than a first threshold, the indicating carried out when DFSO duration is greater than a second threshold, an integration based on inlet air-fuel ratio starting only after the inlet air-fuel ratio reaches stoichiometry. In this way, a measured air-fuel ratio may be used in the integration enabling a reduction in noise and errors.
For example, a catalyst monitor may be activated after a DFSO event in an engine. A catalyst in the engine may store oxygen during the DFSO event and the catalyst monitor may compute an amount of fuel mass required to react with the stored oxygen. As such, the fuel mass may signal an age of the catalyst. The amount of fuel mass may be calculated as the fuel delivered to the catalyst from a first time when a pre-catalyst sensor indicative of inlet air-fuel ratio reaches stoichiometry until a second time when a post-catalyst sensor reaches a first threshold. The first threshold, in one example, may be stoichiometry. An integration calculation may be utilized to compute the fuel mass, the integration calculation starting from the first time and ending at the second time. Thus, an amount of fuel delivered to the catalyst before the pre-catalyst sensor reaches stoichiometry may be disregarded and may not be included in the integration calculation.
In this way, the integrated air-fuel ratio method may be utilized to detect a catalyst that has reached its threshold use in a more reliable manner. By commencing the calculation when the pre-catalyst sensor reaches stoichiometry, only the amount of fuel that reacts with stored oxygen in the catalyst may be estimated. Further, by using a measured inlet air-fuel ratio instead of a calibrated air-fuel ratio, the calculation may be less sensitive to noise and may provide a more robust catalyst monitoring method. Accordingly, a more accurate prediction of a state of the catalyst may be achieved. As such, erroneous characterization of a degraded catalyst as a useful catalyst (and vice versa) may be reduced. Overall, expenses associated with such errors may be reduced, and emissions may be lowered.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.